Digital screening typically involves superimposing onto digital image data a collection of data representing a screen to produce a digital output that is representative of a screened image. U.S. Pat. No. 4,413,286, issued for an invention of Boston, which is hereby incorporated herein by reference, discloses a digital technique for obtaining a screen image at variable screen angles. The Boston approach in one embodiment involves storing in an array (which there and herein is called a "screen cell" matrix) a series of optical density values representative in each case of the optical density in a specific location within a single screen cell. A given storage address in the array corresponds to a specific location within the screen cell. These values have importance since the screen is formed simply of contiguous repetitions of the screen cell.
In order to reproduce in raster form an image that has been subjected to the screen, Boston begins with the optical density values of the image at address locations identified by reference to a coordinate system, for example, the line number and pixel number of each pixel. The problem is then how to compare each pixel of image data with the optical density in the applicable screen cell location. The problem is nontrivial because the principal axis of the screen does not necessarily coincide with either the line number (i.e., y) axis or the pixel number (i.e., x) axis. Furthermore, the physical size of the screen cell or of the spacing between address locations within the cell for its density data does not generally match the spacing between adjacent pixels or adjacent lines.
Boston teaches that the optical density of the screen at a given set of coordinates of the image data can be determined by computing the pertinent address in the screen cell and accessing the optical density value stored there. Once the screen density at a given set of coordinates of the image data is known, it is a straightforward matter to compare the image pixel density at the same coordinates. Boston further teaches that a method for computing the pertinent screen cell address for each image coordinate is to determine an initial screen cell address for a given image pixel address and thereafter for each increment in pixel number or line number to determine the corresponding increments in the screen cell address coordinates. See column 4, especially lines 35-45.
One embodiment of Boston's invention takes advantage of these teachings by processing image data one pixel at a time, performing the calculation to identify for such pixel the corresponding screen cell address, accessing from the screen cell array the pertinent optical density of the screen, comparing the screen density with the image pixel density, and generating an appropriate output based on the comparison. See column 4, line 45, through column 5, line 34.
The approach of Boston permits screening at any angle and accomplishes it with modest memory requirements (for example, screen cell memory of 256.times.256 elements or even 64.times.64 elements). However, the speed of the screening is affected by the time for calculation of screen cell addresses, and the use of screen cell memory in the size and manner described makes difficult the use of some screen frequencies with certain pixel and line frequencies.
U.S. Pat. No. 4,350,996, issued for an invention of Rosenfeld, discloses the addition of a random number to each screen memory address calculation as a way of reducing the Moire pattern. Column 6, lines 6-14. However, there is a problem in that the greater the amount of noise added to the system the less resolution is available in its output.
U.S. Pat. No. 4,149,194, issued for an invention of Holladay, discloses the use of a repetitive rectangular matrix representative of a screen cell in accomplishing the screening. However, the approach taught there apparently requires advance calculation of specific conditions permitting the repetition to occur, because not all screen angles can be accommodated. Column 7, lines 18-36.